Thermopile and method of making

ABSTRACT

Method of making a thermopile 61 composed of a large number of wire elements in whose practice continuous wires 37 and 41 of dissimilar thermoelectric material are lapped in opposite directions. The wires form an array of regions of intersection. An electric arc is passed along this array while the portions of the wires near the intersection are cooled. The arc produces welded joints 58, 59 and at the same time separates the wires at the joints from the continuum of wire to produce thermocouples 63. A thermopile assembly 80 formed by embedding the wires in insulating sheets 71, 73 before they are welded and wrapping the sheet with the wires in them in a cylinder 91. Apparatus for making the thermopile including a mandrel 11 with opposite running helical grooves 13, 15, 17, 19 for winding the wires 37, 41, channels 13, 25, 27, 29 along the arrays of intersection of the wires and chill blocks 51 for engaging the wires near their array of intersections to cool them.

United States Patent 715,265 12/1902 v Heil lnventors Alfred SugarmanMonroeville;

llan Chang, Apollo; Alvin H. Kasberg, Murrysville, all 01 Pa.

Nov. 6, 1967 Sept. 2 1 197 1 Nuclear Materials and Equipment CorporationApollo, Pa.

Appl. No. Filed Patented Assignee References Cited UNITED STATES PATENTSPrimary Examiner-Benjamin R. Padgett Assistant Examiner-Stephen U.Lechert Attorney-Hymen Diamond ABSTRACT: Method of making a thermopile61 composed of a large number of wire elements in whose practicecontinuous wires 37 and 41 of dissimilar thermoelectric material arelapped in opposite directions. The wires form an array of regions ofintersection. An electric arc is passed along this array while theportions of the wires near the intersection are cooled. The are produceswelded joints 58, 59 and at the same time separates the wires at thejoints from the continuum of wire to produce thermocouples 63. Athermopile assembly 80 formed by embedding the wires in insulatingsheets 71, 73 before they are welded and wrapping the sheet with thewires in them in a cylinder 91. Apparatus for making the therrnopileincluding a mandrel 11 with opposite running helical grooves 13, 15, 17,19 for winding the wires 37, 41, channels 13, 25, 27, 29 along thearrays of intersection of the wires and chill blocks 51 for engaging thewires near their array of intersections to cool them.

PATENTED SEPZI I971 3.607.446

SHEU 1 BF 3 I9 IS IS PATENTED SEPZI I97! SHEET 2 [IF 3 FlG.2A.

' FIG.2B.

FIG.4.

PATENTED SEPZI i971 SHEET 3 BF 3 FIG.5A.

FIG.7.

mmisq zz kzwmmbo OUTPUT VOLTAGE THERMOPILE AND METHOD OF MAKINGBACKGROUND OF THE INVENTION This invention relates to the generation ofelectrical power for use in regions of the earth where commercialelectrical power is not readily available or in outer space and hasparticular relationship to the generation of power in the milliwattrange, for example to electrical generators which deliver 100 milliwattsof power. Power generators include a source of heat, for example aradioisotope source, and a thermoelectric converter. Where the generatoroperates in the milliwatt range, the thermoelectric pile is made up of alarge number of thermocouples formed of pairs of wire elements ofdissimilar thermoelectric properties. To attain the desired voltagethese thermocouples are connected in series. It is an object of thisinvention to provide such a thermopile made up of a large number,typically of the order of several hundred, thermocouples.

In accordance with the teachings of the prior art, wires of dissimilarthermoelectric properties are lapped on a rectangular card or core ormetal band and the intersecting ends are formed into junctions byresistance welding or plating. The wires are then severed at the jointsby a cutter to form a plurality of thermoelectric elements. Where a pileof a large number (several hundred) thermoelectric couples is to beproduced this procedure is not practical. The wires cannot readily beremoved from the card, core or band; where an attempt is made to removethem the wires become tangled or knotted and reassembly of the pile withparts cut away is required. This difficulty is increased where the wiresare of small diameter. The forming of the joints or junctions alsopresents severe problems particularly where the wires are of smalldiameter or of high resistivity. Firm electrical connection at thejoints is indispensable and this necessitates that the wires bemetallurgically joined at the joints. But heat adequate to produce afirm electrical connection at a joint of two wires is frequently so highthat the wires near the joint are burned away.

It is an object of this invention to overcome the difficulties anddisadvantages of the prior art in the making of thermopiles formed of alarge number of thermocouples. It is a further object of this inventionto provide a method of making a thermopile of a large number of wires inwhose practice the junctions of the couples of the piles shall bemetallurgical joints constituting firm electrical connection of theindividual wires and the wires at or near the joints shall be sound. Itis also an object of this invention to provide thermopiles made by thismethod. Also it is an object of this invention to provide novelapparatus for practicing this method.

SUMMARY OF THE INVENTION This invention arises from the discovery thatif the individual wires at, or near, an intersection of a pair of wiresare adequately cooled, the wires at this intersection may bemetallurgically joined by welding and at the same time severed. A pairof separate thermoelectric junctions are thus formed in a single stroke.

In accordance with this invention, a thermopile with a large number ofthermocouples is formed by lapping continuous wires of dissimilarthermoelectric properties in opposite direction to produce an array ofintersections, each between a wire of one thermoelectric property and awire of another thermoelectric property. A welding arc is moved alongthe array impinging on the intersections and, at the same time, thewires at or near the intersections are cooled. Typically the welding arcis of the TIG type. The are produces fusion welds of the wires at theintersection and in addition severs the joints so that two junctions areproduced at each intersection.

In accordance with this invention the wires are lapped on a cylindricalmandrel which has oppositely running helical grooves to accommodate therespective wires of the dissimilar thermoelectric properties. Forcooling purposes the mandrel is composed of a highly thermallyconducting material, typically copper or silver. Channels, parallel tothe axis of the mandrel, are provided along the intersections of theoppositely running grooves so that the welding operation may beeffectively carried out.

The cooling is effected by chill blocks which extend over the mandreland have projections or edges engaging the wires at or near theintersections in cooling relationship.

After being wound on the mandrel the wires of each thermoelectricproperty are secured to electrically insulating sheets which leaveexposed short sections of the wires, at the intersections. Thesesections are fusion welded. The sheets and wires form a thermopileassembly which is rolled into a cylinder that may be used in a generatorwith facility.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of thisinvention, both as to its organization and as to its method ofoperation, together with additional objects and advantages thereof,reference is made to the following description taken in connection withthe accompanying drawings, in which:

FIG. l is a view in perspective showing apparatus in accordance withthis invention used in the winding of wire in the practice of thisinvention;

FIG. 1A is an enlarged view of the section IA of FIG. 1;

FIG. 2 is an exploded view in perspective showing use of apparatus inaccordance with this invention in the making of a thermopile from woundwires;

FIGS. 2A and 2B are enlarged views showing the formation of junctions orjoints of wires of dissimilar thermoelectric properties in the practiceof this invention;

FIG. 3 is an exploded view similar to FIG. 2 but showing the insulatingsheets to which the wires of dissimilar thermoelectric properties aresecured;

FIG. 4 is a plan view of a thermopile formed with the apparatus shown inFIG. 3;

FIG. 5 is a view in end elevation of the pile shown in FIG. 4;

FIG. 5A is an enlarged view of section VA of FIG. 5;

FIG. 6 is a view in side elevation ofthe pile shown in FIG. 4;

FIG. 7 is an exploded view in perspective of the pile shown in FIG. 4assembled in a cylinder; and

FIG. 8 is a graph showing the actual operation of the pile shown in FIG.7.

DESCRIPTION OF PREFERRED EMBODIMENTS AND PRACTICE The apparatus shown inthe drawings includes a cylindrical mandrel 11 composed ofa materialhaving high thermal conductivity. A plurality of pairs of grooves 13 and1S and 17 and 19 extend around the surface of the mandrel in helices.The grooves !l3 and 15 have a clockwise, or left-hand, direction ofrotation, as viewed from one end of the mandrel, as they advance alongthe surface and the grooves 17 and 19 have a counterclockwise, orright-hand, direction of rotation viewed the same way. Typically thestarting points for the grooves l3, 15, 17 and 19 are displaced byaround the cross section of the mandrel. The pitch of the helices may beset so that there are a selected number of arrays of intersections ofthe helices; typically, four linear arrays of regions of intersection 21of the clockwise and counterclockwise grooves 13 thru 19 spaced by 90around the periphery of the mandrel 11 are shown. The mandrel II isprovided with channels 23, 25, 27, 29 (FIG. 3) along these arrays.

The apparatus further includes means for rotating the mandrel 11 whichtypically is a lathe 31. The mandrel 11 is mounted between the chuck 33and spindle 35 of the lathe 31 and is rotated as the chuck 33 is driven.

A wire 37 of desired thermoelectric properties is wound from a reel 39on the mandrel 11 as the mandrel is rotated. The wire 37 is deposited inone of the grooves, 13 or 15 for example. The grooves 13 or I5 aredimensioned to accommodate the wire 37 readily; the depth of the groovesshould be somewhat greater than the diameter of the wire so that thewire fits neatly in the grooves.

The wire 37 is then secured to the mandrel 11 so that it remains tightlywound on the mandrel. If, as is typical, the thermopile is to includecouples connected in parallel in double redundancy, a second wire (notshown) of thermoelectric properties similar to wire 37 is wound on thismandrel in the other groove 15 or 13 and secured tightly wound to it.Next a wire 41 of dissimilar thermoelectric properties from wire 37 iswound in the cross grooves 17 or 19 on opposite rotation grooves. Inthis case also a second wire of same thermoelectric properties as wire41 may be wound in grooves 19 or 17, as the case may be, where the pileis to include couples connected in double redundancy. The wires 37 and41 intersect in arrays along the channels 23, 25, 27 and 29.

With the wires 37 and 41 wound on, and secured to, the mandrel 11 themandrel is removed from the lathe 31. The procedure just described maybe carried out with enameled wires 37 and 41, the enamel insulating thewires electrically from each other. It is now necessary to prepare thewires on the mandrel 11 for welding. For this purpose the mandrel ismounted securely between the brackets 43 and 44 of a supporting fixture45 and the segments between the channels 23 thru 29 are covered leavingexposed only the regions of intersection of the wires 37 and 41. Theregions of intersection are then sandblasted to remove the enamel fromthe exposed portions of the wires and the sandblasted areas arethoroughly cleaned.

The apparatus includes a plurality of chill blocks 51 (only one shown)cooperative with the mandrel 1 1 on the fixture 45. Each chill block 51is composed of a material of high thermal conductivity, such as copper,and is of generally C-shape having legs 53 whose tips 55 are beveled.The chill blocks 51 are secured to the mandrel l 1 and extend acrosseach quadrant of its surface (where there are four arrays of regions ofintersection). The length of each chill block 51 is such that it extendsover the wires 37 and 41 on the surface of the mandrel 11 with the tipsof the bevels 55 on the legs 53 engaging the wires 37 and 41 at or neartheir intersections. The chill blocks 51 are secured to the mandrel 11so that this engagement is firm assuring effective transfer of heatbetween the intersections of the wires and the blocks 51. With the chillblocks 51 secured to the mandrel 11 and the chill block mandrel assemblyin the supporting fixture 45, the wire intersections along each of thearrays are fused into junctions and separated. For this a TlG weldingelectrode 56, (FIG. 2A) appropriately connected to a welding supply, isset into welding relationship with an array 57 of intersections at oneend of each slot 23, 25, 27, 29 in its turn. An arc is fired between theelectrode and the wires and the arc is moved along each array infusion-welding relationship with the array 57. The heat of the arccauses the wires 37 and 41 at the intersection to be metallurgicallyjoined and in addition separates the wires at each joint into twoseparate pairs each having a welded tip of junction 58 and 59 (FIG. 2B).The chill blocks 51 conduct away the excess heat from the jointspreventing damage to, or burning of the wires at, or near, the joints.By welding and separating in this manner the arrays of intersectionsalong adjacent slots 23 and 25, for example, a pile 61 ofseries-connected thermocouples 63 are formed; each couple has a lengthsubstantially equal to onefourth the circumference of the mandrel andthis length determines the electrical resistance of the couple. Wherethe mandrel has only a single slot a pile may be formed of couples eachof which has a length substantially equal to the circumference of themandrel. The pile so formed may be mounted on or secured to a frame orcloth and used in any desired manner.

Where the wires 37 and 41 are bare, sheets 71 and 73 of electricalinsulating material may be secured to, or disposed on, each array ofwires as shown in FIG. 3. in this case wire 37 is first wound tightly ona mandrel 11 and secured to the mandrel. Then, insulating sheets 71having a width such that they cover the turns of wire 37 betweenadjacent slots 23 and 25, and 27, 27 and 29, and 29 and 23 are disposedon or secured to the layer of wires. The sheets 71 may be adhesive tapewith adhesive on both faces and they are adhesively secured to the wires37 and 41. The sheets 71 and 73 may also be sprayed on. Between thesheets 71 short sections of wire 37 extending over the slots 23 to 29are exposed.

The wire 41 is then tightly wound on sheet 71 with the mandrel 11rotated in the opposite direction during the winding. Where there isadhesive on the outer face of tape 71 wire 41 is secured to thisadhesive. The slots 17 and 19 are covered by the sheets 71 but thewinding of wire 41 is guided by the exposed short sections of the wire37. In addition the lathe 31 is set so that the pitch of the wire 41 isproperly maintained. The winding of the wire 41 is started so that theintersections of wires 37 and 41 extend along slots 23 thru 29.

Next with wire 41 secured to the mandrel 75, sheets 73 similar to sheets71 are disposed on wire 41. The joints between wires 37 and 41 are thenformed by arc welding as described above.

Where the thermopile is to be used in a vacuum, typical adhesivesreadily available decompose when subjected to the treating-outtemperature during evacuation (700 C.). In this case the tape-pileassembly made as disclosed above is first sewed with quartz thread andthen the adhesive is dissolved away. Usually the pile is secured to thetapes by stitches along the junctions and near the center of the pile.

A thermoelectric assembly in accordance with this invention is shown inF105. 4 thru 6. The pile 61 is secured to the tape 71, 73 whichinsulates the wires 37 and 41 of the couples 63 from each other.Terminals 81 are welded to the end wires 83 at both ends (one end notshown).

As shown in H0. 7 the assembly 80 is wrapped into a cylinder 91. Theprotruding junctions are then encased in ceramic potting compound(typically aluminum phosphate Al(PO:;).-;) which has high thermalconductivity. The compound is formed into bases at 93 at the ends whichtaper out from the cylinder 91. Ceramic discs 95 are brazed (brazingcompound Cu 15.6 Ti 66.3 Ni 18.1) to the bases 93 and metal discs 97,typically, of columbium coated with copper are brazed (same compound) tothe ceramic discs 95. The terminal 81 that is connected to the negativeend of the pile 61 is grounded; the other terminal is ungrounded.

The heat source is applied to the one end disc 97, which does notinclude the terminals 81. The other end disc is connected to the heatsink.

The following description of actual making of a thermopile assembly mayaid in the understanding of this invention.

A copper (sulfur copper) mandrel 11 was placed in a lathe 31 for whichthe centering hole 99 on one end of the mandrel was made. The mandrel 11is 1.592 inches in diameter and 22 inches in length with two left-handhelical grooves 13 and 15 and two right-hand helical grooves 17 and 19each with Vainch pitch and a /a-inch lead. The grooves are 000710.001inch by (100710.001 inch.

One pair of 3% mil chromel wires 37 were fed into each of the two setsof helical grooves at about 12 rpm, after which a l fii-inch wide fiberglass tape 71 is sprayed with a rubber cement compound and applied overthe grooves and wires. The above winding procedure is then repeated with3% mil constantan wire which is wound over the fiber glass tape 71 inthe opposite direction, and again covered with a l%.-inch fiber glasstape 73 coated with a rubber cement.

The copper chill blocks 51 are then screwed on and the fixture is nowready to accommodate the welding process.

The welding is carried out with a tungsten inert gas welder using al/l6-inch tungsten electrode 56 with nitrogen purging gas at about l ampwelding current. No slope control or time control is used. The electrodeis held in the hand and the ceramic cup, from which the shielding gasflows, is rested on the edges of each consecutive pair of blocks 51 sothat the electrode tip-wire distance can be adjusted and be maintainedsteady with the hand. After the arc is fired at the extreme of themandrel 11 with the high frequency stabilizer, the arc is drawn down thelength of the channel 23 that is machined in the surface of the mandrel25, 27, 29. This procedure is repeated in each of the three remainingchannels so that all the thermocouples are welded. It has been foundthat the are power may be varied over a reasonable range withoutaffecting the soundness of the welding.

Upon completion of the welding, the blocks 51 are removed and the fiberglass tapewire-fiber glass tape assembly 80 is peeled from the surfaceof the mandrel. The rubber cement causes the thermocouple wires toadhere to the glass tape while not sticking to the copper mandrel. Theassembly may then be sewn together and the adhesive dissolved away.

The following brief summary may further aid in the understanding of thisinvention.

This invention relates to the thermopiles for radioisotope generatorsoperating in the milliwatt range; a typical generator operates at 100M.W. In making this thermopile it was necessary to provide a (1) methodfor joining the thermocouple wires 37 and 41; (2) a means of insulatingeach of the thermocouple wires from each other. 7 V

In typical practice in accordance with the teachings of the prior art, athermopile consisting of 1,300 thermocouples made from Tophel Specialand Cupron Special wires of 2 mils diameter was made. The hot and coldjunctions were made by twisting, dipping, and brazing the wire ends. Theentire thermopile was potted in silicone rubber. The thermopile wasdesigned to operate with a cold side of 100 F. (38 C.) and a hot side of400 F. (204 C.) to produce 1.55 milliwatts. The making of thisthermopile presented severe difficulties and consumed excessive time andthe pile was very costly.

in the practice of this invention it was necessary to provide athermopile to operate with a cold-junction temperature at 200 C. and ahot-junction temperature of 600 C., to generate 100 milliwatts.

This invention is described in detail above. In the specific practice ofthis invention, a solid l'h-inch copper cylinder 1 1, 22 inches inlength on the surface of which are threads or helical grooves 13 and 15and 17 and 19. The threads 13, 15, 17, 19 are two 0.005 inch by 0.005inch grooves 13, 15 for 0.003- inch wires 37 in double redundancy,threaded in the righthand direction with a 18 inches advance and two ofthe same grooves 17, 19 in the left-hand direction with the same advance. The grooves are started 90 from each other and meet on four linesparallel to the axis of the cylinder on which is machined a l/ 16-inchwide by 0.010-inch deep channels 23, 25, 27, 29 in which the wires arewelded. To make the thermocouples, the wire 37 (or wires, whenredundancy is desired) are wound in the grooves 13, 15 in one directionfor one leg of the thermocouple and 49 in the other direction for theother leg of the thermocouple. The winding can be by hand or with alathe 31 in which the wires are fed through a guide 101 into the groovesas the cylinder turns on its axis and the feeder follows a particulargroove. The wires are then welded along their intersections.

The apparatus includes the chill blocks 51 which engage the wires neartheir intersections in cooling relationship. It is important that theblocks 51 and cylinder 11 are flush along the cylinder surface andchannel edges to present a uniform heat ,sink to the weld bead and toprevent the head from flowing between the cover and cylinder. It is alsoimportant for all the wires to be together at the weld area and not lessthan l/65 inch behind the weld area to prevent the occurrence of brokenand unwelded wires.

Welding is carried out by the T16 process in which a fine tungstenelectrode is held about 1116 inch above the center of the section ofwires bridging the l/ l 6-inch channel and moved in the axial direction,after an arc is fired, at a rate of about 30 feet/minute.

The blocks 51 are hollow so that tape 71, 73 can be placed over the barewires and not interfere with the thermal contact among the block, wiresand cylinder 11. After the welding is completed, the covers can beremoved and the wires peeled from the mandrel with the tape 71, 73. Inthis way the positions of the thermocouple legs are not altered inhandling.

The factors important in achieving a high degree of reliability in thepractice of our invention are:

1. Good thermal contact between the blocks 51, wires 37, 41 and cylinder11.

2. A flat heat sink that cools the beads uniformly.

3. Grooves 13, 15, 17, 19 in which the wires are wound, that are largeenough to keep the wires in place and together but small enough to allowthe cover to exert a minimum of pressure on them.

4. Uniform but small wire tension.

5. Channels 23, 25, 27, 29 that are deep enough so that wire-metalcontact is not made and narrow so that the bead of molten metal does notcool before making contact with the heat sink.

Important features of this invention are:

1. That a pile of a large number of series connected thermocouples maybe made by welding the wires to each other, thereby eliminating a brazematerial.

2. That the thermocouple 63 may be readily insulated from each other sothat the thermopile 61 can operate at the temperature capability of thethermocouple material.

3. That wires of any gauge, over a reasonable range, can be weldedtogether to make a thermopile.

4. That any reasonable number of wires can be welded to each other.

5. That a pile of any reasonable number of units of thermocouplesconnected in series can be fabricated.

6. That any reasonable size of thermopile can be made.

FIG. 8 shows the characteristics of a thermopile according to thisinvention operating between hot-junction and coldjunction temperaturesof 435 C. and 50 C. and 500 C. and 50 C., respectively. This piledelivered 15 milliamps at about 6.7 volts at the lower hot-junctiontemperature and 15 milliamps at 9.8 volts at the higher temperature.This thermopile was composed of 416 couples of Tophel Spe cial wire (NiCr 10) and Cupron Special wire (Cu 60 Ni 39.5 Mn 0.25).

We claim as our invention:

1. The method of making a thermopile composed of a large number ofthermoelectric elements which comprises lapping continuous wires ofdissimilar thermoelectric material in opposite directions so that thewires cross over at least once producing at least one array of regionsof intersection of said wires, impinging an electric are on said regionsof intersection and at the same time conducting away the heattransferred by said arc through said regions from the portions of thewires forming said regions, both, to produce fusion at each region ofintersection and said fusion severing the wires forming saidintersection to produce at each said region at least one thermoelectricelement having a fusion joint as a junction and the dissimilar wiresremaining connected to said joint as com ponents the wires of eachthermoelectric material being insulated from contiguous wires ofdissimilar thermoelectric material except at the fusion joints.

2. The method of claim 1 wherein the wires cross over at least twiceproducing at least two arrays of regions of intersection on each ofwhich array an arc is impinged.

3. The method of claim 1 wherein the wires are lapped by being wound ona mandrel of high thermal conductivity material.

4. The method of claim 1 wherein heat is conducted away from theportions of the wires forming the regions of intersection by a chillblock which engages said wires near said portions in coolingrelationship.

5. The method of claim 1 wherein the wires of dissimilar thermoelectricproperties are enameled whereby contiguous wires of each thermoelectricmaterial are insulated from contiguous wires of dissimilarthermoelectric material by enamel on the surface of at least a wire ofone of said materials.

6. The method of producing a thermopile composed of a large number ofthermoelectric elements which comprises winding a wire of onethermoelectric material on a mandrel in a helix in one direction,covering said wire with electrically insulating material leaving exposedshort sections of wire of each of the turns of said helix, then windinganother wire of another thermoelectric material dissimilar from said onematerial on said mandrel over said insulating material in another helixin a direction opposite to said one direction, such that said other wireforms crossover regions with said first-named wire at the exposed shortsections of said firstnamed wire covering said other wire withadditional electrically insulating material, fusing the wires of saidone and other thermoelectric material together at said crossover regionsto form fused joints at said regions, and said fusion severing saidwires at said fusion joints to produce at each said region at least onethermoelectric element having a fusion joint as a junction and thedissimilar wires remaining connected to said joint as components.

7. The method of claim 6 wherein the mandrel has at least one channelextending in an axial direction along the surface on which the wires arewound and the wires are wound so that the crossover regions extend alongsaid channel.

8. A thermopile comprising a first plurality of wires of a firstthermoelectric material covered with a first layer of insulatingmaterial having exposed short sections of said wires along at least onelength of said insulating material, a second plurality of wires of asecond thermoelectric material, dissimilar from said first material,covered with a second layer of insulating material disposed over saidfirst layer of insulating material and having exposed, short sections ofsaid wires of said second plurality along at least one length of saidinsulating material. said last-named short sections fused to saidfirst-named short section and severed to produce at each end section atleast one thermoelectric element having a fusion joint as a junction andthe dissimilar wires remaining connected to said joint as components.

9. The thermopile of claim 8 wherein the wires and insulating materialare in the shape of a cylinder.

2. The method of claim 1 wherein the wires cross over at least twiceproducing at least two arrays of regions of intersection on each ofwhich array an arc is impinged.
 3. The method of claim 1 wherein thewires are lapped by being wound on a mandrel of high thermalconductivity material.
 4. The method of claim 1 wherein heat isconducted away from the portions of the wires forming the regions ofintersection by a chill block which engages said wires near saidportions in cooling relationship.
 5. The method of claim 1 wherein thewires of dissimilar thermoelectric properties are enameled wherebycontiguous wires of each thermoelectric material are insulated fromcontiguous wires of dissimilar thermoelectric material by enamel on thesurface of at least a wire of one of said materials.
 6. The method ofproducing a thermopile composed of a large number of thermoelectricelements which comprises winding a wire of one thermoelectric materialon a mandrel in a helix in one direction, covering said wire withelectrically insulating material leaving exposed short sections of wireof each of the turns of said helix, then winding another wire of anotherthermoelectric material dissimilar from said one material on saidmandrel over said insulating material in another helix in a directionopposite to said one direction, such that said other wire formscrossover regions with said first-named wire at the exposed shortsections of said first-named wire covering said other wire withadditional electrically insulating material, fusing the wires of saidone and other thermoelectric material together at said crossover regionsto form fused joints at said regions, and said fusion severing saidwires at said fusion joints to produce at each said region at least onethermoelectric element having a fusion joint as a junction and thedissimilar wires remaining connected to said joint as components.
 7. Themethod of claim 6 wherein the mandrel has at least one channel extendingin an axial direction along the surface on which the wires are wound anDthe wires are wound so that the crossover regions extend along saidchannel.
 8. A thermopile comprising a first plurality of wires of afirst thermoelectric material covered with a first layer of insulatingmaterial having exposed short sections of said wires along at least onelength of said insulating material, a second plurality of wires of asecond thermoelectric material, dissimilar from said first material,covered with a second layer of insulating material disposed over saidfirst layer of insulating material and having exposed, short sections ofsaid wires of said second plurality along at least one length of saidinsulating material, said last-named short sections fused to saidfirst-named short section and severed to produce at each end section atleast one thermoelectric element having a fusion joint as a junction andthe dissimilar wires remaining connected to said joint as components. 9.The thermopile of claim 8 wherein the wires and insulating material arein the shape of a cylinder.